Even the new formula is wrong and I've written about this to numerous people in the industry, but they still don't seem to get it.
Ammonia
The original 10x rule came from the relationship of how much chlorine it took to oxidize ammonia. The net equation is as follows for breakpoint chlorination:
3HOCl + 2NH
3 ---> N
2(g) + 3H
2O + 3H
+ + 3Cl
-
Hypochlorous Acid + Ammonia ---> Nitrogen Gas + Water + Hydrogen Ion + Chloride Ion
So on a molar basis, the ratio of chlorine to ammonia is 3 to 2 or 1.5 to 1. To get to a ppm basis, one must convert to chlorine units which are measured as ppm Cl
2 and to ammonia units which are measured as ppm N. The molecular weights for these are 70.9064 and 14.0067, respectively. So on a ppm basis, the ratio is 1.5*70.9064/14.0067 = 7.59. In practice due to some side reactions and needing a little extra to get over the hump of the multiple initial steps for the reaction, it takes 8 to 10x the amount of chlorine as ammonia. This is where the 10x came from.
However, when it comes to combined chlorine in water, there are two problems with using the 10x rule. First of all,
combined chlorine (CC) is measured in chlorine units, not ammonia units, so the factor of 5 difference in these units needs to be taken out. The research referenced in the Taylor article STILL doesn't recognize this flaw of measuring units. Second, combined chlorine already has used up one of the 1.5 chlorine needed in the breakpoint chlorination reaction as seen from the first step of that reaction:
HOCl + NH
3 ---> NH
2Cl + H
2O
Hypochlorous Acid + Ammonia ---> Monochloramine + Water
The research referenced in the Taylor article is only accounting for this latter effect, which is relatively minor compared to the factor of 5 units effect. So in practice, in the worst case where you had monochloramine (combined chlorine), you would have to add a little more than the same amount of chlorine in order to get the breakpoint reaction going (that gets it to dichloramine where any more chlorine starts breaking it down and releasing more chlorine).
So that's 1x, not 10x where the actual amount of chlorine used would be 0.5x (i.e. there would be chlorine leftover).
Urea
Now what I have described above is for ammonia, but the largest component of sweat and urine is urea and that requires more chlorine to get oxidized as shown below (this model is not definitive, but proposed by Wojtowicz and is roughly consistent with data seen to date; hopefully a more accurate or confirmed model will be created by Blatchley's research):
(NH
2)
2CO + 4HOCl ---> (NCl
2)
2CO + 4H
2O
Urea + Hypochlorous Acid ---> Quadchlorourea + Water
(NCl
2)
2CO + HOCl ---> NCl
3 + NHCl
2 + CO
2
Quadchlorourea + Hypochlorous Acid ---> Nitrogen Trichloride + Dichloramine + Carbon Dioxide
NCl
3 + NHCl
2 + 2H
2O ---> 2HOCl + N
2(g) + 3H
+ + 3Cl
-
Nitrogen Trichloride + Dichloramine + Water ---> Hypochlorous Acid + Nitrogen Gas + Hydrogen Ion + Chloride Ion
--------------------------------------------------------------------------------------------
3HOCl + (NH
2)
2CO ---> N
2(g) + CO
2 + 2H
2O + 3H
+ + 3Cl
-
Hypochlorous Acid + Urea ---> Nitrogen Gas + Water + Carbon Dioxide + Hydrogen Ion + Chloride Ion
So on a molar basis it takes 3 times as much chlorine as urea. If we assume the worst case where a monochlorourea was what was getting measured as combined chlorine, then it would take at most 3 more chlorine to get the breakpoint reaction started (this gets it to quadchlorourea where any additional chlorine starts breaking that down and releasing more chlorine).
So that's 3x, not 10x where the actual amount of chlorine used would be no more than 2x (i.e. there would be chlorine leftover). In practice, it would take less chlorine than 3x since some of the combined chlorine is probably at least dichlorourea.
By the way, the chloroureas may not be as volatile nor irritating so they may just be wasteful readings of Combined Chlorine (CC) that just indicate intermediate compounds that aren't a serious problem (until they get rather high). If you've got a situation with CC where there isn't any obnoxious or noticeable pool smell, then the CC may be something that can be ignored, though it could indicate too slow an oxidation of organics in your pool. Not every chlorine compound is a problem, even though some are. Unfortunately, the current tests don't distinguish between these very well, though an ammonia test kit could potentially be used to measure monochloramine specifically separate from other combined chlorines (I need to confirm this).
Disinfection By-Products
As for higher chlorine levels producing more disinfection by-products, this is theoretically true for nitrogen trichloride as I describe in
this post, but it is not necessarily true for other disinfection by-products and in fact lower active chlorine levels result in higher monochloramine and dichloramine levels though these are temporary unless there is continual bather load.
Getting "Stuck" is a Fallacy
There is also this incorrect information in the industry that somehow if you add insufficient levels of chlorine you then get "stuck" with intermediate by-products you cannot get rid of. This is simply not true. If you do not use sufficient chlorine, then you do get intermediate by-products such as monochloramine or a chlorourea, but you can simply add more chlorine and continue the reactions from that point. Nothing gets "stuck".
In practice, if one always maintains some level of Free Chlorine (FC) in the pool at all times, then the breakpoint reactions are occurring continuously, though not necessarily as quickly as one may want.
Reaction Rates
Note that the above 1x or 3x rules simply refer to the minimum amount of Free Chlorine (FC) target needed to get the breakpoint reaction going in the worst case. It says nothing about the speed of that reaction. With CYA in the water, the active chlorine level is quite low so the speed of the breakpoint reaction can be slow as well. For ammonia, this time can be readily predicted. At 77F and with the FC at around 10% of the CYA level, it takes around 3-1/2 hours for the oxidation of ammonia to be 90% complete. If there were 2 ppm FC with no CYA in the water, then 90% completion would take around 15 minutes (but with nearly 20 times the amount of nitrogen trichloride produced). The oxidation of urea can take far longer, even days, though it is apparently helped by the UV in sunlight which partly explains why there is usually less Combined Chlorine (CC) in residential outdoor pools than indoor pools. The oxidation of urea is also apparently very temperature dependent, which explains why the CC doesn't usually build up in residential spas that are even used daily.
The good news with the slower oxidation speed when there is CYA in the water is that there is less nitrogen trichloride produced and that is the most irritating and volatile of the disinfection by-products (though not necessarily the most harmful -- chloroform could be worse, but comes from other organics).
Commercial/public pools and spas with high bather loads have a bigger problem with CC and disinfection by-products, but this is very much a function of the bather load and there isn't much that can be done about this without supplemental oxidation (ozone, UV, enzymes, non-chlorine shock such as MPS, etc.) or significant water replacement.
So this entire idea of needing some factor of the CC as chlorine to be added is ridiculous from the point of view of "needing" that much chlorine. Since there is usually measurable FC in the water at the same time there is CC, the only purpose for increasing the FC is to increase the reaction rates to oxidize things faster. It isn't because it is "needed". There is no discussion in the industry about the FC/CYA ratio which is what determines the active chlorine concentration and therefore the reaction rates. Again, the industry focuses solely on FC alone which is relevant for total amount of chlorine needed to complete the reaction, but is irrelevant in regularly dosed pools maintaining a certain FC level.
Richard